In 1899, the Serbian inventor Nikola Tesla noticed strange rhythmic sounds on his radio receiver in his Colorado Springs laboratory. The sounds were in such a regular pattern that Tesla concluded it could only be an effort to communicate with Earth by alien beings. The signals were certainly extraterrestrial: radio waves from distant celestial sources – planets, comets, stars or galaxies.

This was confirmed by scientists in 1932, and gave birth to the field of radio astronomy. By analyzing the radio signals emitted by celestial sources, astronomers are detecting and decoding their own messages from distant stars.

More than 30 years later, in 1967, a young female graduate student in astronomy named Jocelyn Bell noticed a strange “bit of scruff” in the data coming from her radio telescope at the Mullard Radio Astronomy Observatory (MRAO) near Cambridge. Bell was using a new technique, called interplanetary scintillation, to observe quasars (QUASi-stellAR radio sources), the most distant single objects observed in the universe to date, believed to be found at the centers of young galaxies.

Because quasars scintillate more than other objects – that is, they emit much larger amounts of light and other forms of electromagnetic radiation, despite their relatively small size — Bell’s advisor, Anthony Hewish, thought the technique would be a good way to study them, and he designed a radio telescope to do so.

Bell spent about two years building the new telescope at MRAO, with the help of several other students. Together they hammered over 1000 posts, strung over 2000 dipole antennas between them, and connected it all up with 120 miles of wire and cable. The finished telescope covered an area of about four and a half acres.

They started operating the telescope in July 1967. Bell had responsibility for operating the telescope and analyzing the data–nearly 100 feet of paper every day–by hand. She soon learned to recognize scintillating sources and interference.

Within a few weeks Bell noticed something odd in the data. The signal didn’t look quite like a scintillating source or like manmade interference. She soon realized it was a regular signal, consistently coming from the same patch of sky.

No known natural sources would produce such a signal. Bell and Hewish began to rule out various sources of human interference, including other radio astronomers, radar reflected off the moon, television signals, orbiting satellites, and even possible effects from a large corrugated metal building near the telescope. None of those could explain the strange signal.

The signal, a series of sharp pulses that came every 1.3 seconds, seemed too fast to be coming from anything like a star. Bell and Hewish jokingly called the new source LGM-1, for “Little Green Men.” (It was later renamed.) Jokes aside, there was the very real possibility that they had detected a signal from an extraterrestrial civilization.

But soon they managed to rule out extraterrestrial life as the source of the signal, when Bell noticed another similar signal, this time a series of pulses arriving 1.2 seconds apart, coming from an entirely different area of the sky. It seemed quite unlikely that two separate groups of aliens were trying to communicate with them at the same time, from completely different locations. Over Christmas 1967, Bell noticed two more such bits of scruff, bringing the total to four.

By the end of January 1968, Bell and Hewish submitted a paper to the scientific journal, Nature, describing the first pulsar (short for pulsating star). In February, a few days before the paper was published, Hewish gave a seminar in Cambridge to announce the discovery, though they still had not determined the nature of the source.

The announcement caused quite a stir. The press jumped on the story–the possible finding of extraterrestrial life was too hard to resist. They became even more excited when they learned that a woman was involved in the discovery. Bell later recalled the media attention in a speech about the discovery: “I had my photograph taken standing on a bank, sitting on a bank, standing on a bank examining bogus records, sitting on a bank examining bogus records. Meanwhile the journalists were asking relevant questions like was I taller than or not quite as tall as Princess Margaret, and how many boyfriends did I have at a time?”

Other astronomers were also energized by the finding, and joined in a race to discover more pulsars and to figure out what these strange sources were. By the end of 1968, dozens of pulsars had been detected. Today astronomers believe them to be spinning neutron stars, which is why they give off bursts of radio waves at regular intervals.

After discovering the first pulsars, Jocelyn Bell finished her analysis of radio sources, completed her PhD, got married and changed her name to Burnell. She left radio astronomy for gamma ray astronomy and then x-ray astronomy, though her career was hindered by her husband’s frequent moves and her decision to work part time while raising her son. Hewish won the Nobel Prize in 1974 for the discovery of the first pulsars. Over 1000 pulsars are now known.

As for little green men, they haven’t been found yet, but projects such as the Search for Extra Terrestrial Intelligence (SETI) are still looking for them.